Par means age of Bramha--the passionate(rajoguni) creator of universe. Parardha means its half. See following description from Bhagvata Purana Canto 3, Chapter 11.
hy anāder jagad-ātmanaḥ
kālaḥ — eternal time; ayam — this (as measured by Brahmā's duration of life); dvi-parārdha-ākhyaḥ — measured by the two halves of Brahmā's life; nimeṣaḥ — less thana second; upacaryate — is so measured; avyākṛtasya — of one who is unchanged; anantasya — of the unlimited; hi — certainly; anādeḥ — of the beginningless;jagat-ātmanaḥ — of the soul of the universe.
The duration of the two parts of Brahmā's life, as above mentioned, is calculated to be equal to one nimeṣa [less than a second] for the Supreme Personality of Godhead, who is unchanging and unlimited and is the cause of all causes of the universe.
The great sage Maitreya has given a considerable description of the time of different dimensions, beginning from the atom up to the duration of the life of Brahmā. Now he attempts to give some idea of the time of the unlimited Personality of Godhead. He just gives a hint of His unlimited time by the standard of the life of Brahmā. The entire duration of the life of Brahmā is calculated to be less than a second of the Lord's time, and it is explained in the Brahma-saḿhitā (5.48) as follows:
"I worship Govinda, the Supreme Personality of Godhead, the cause of all causes, whose plenary portion is Mahā-Viṣṇu. All the heads of the innumerable universes [the Brahmās] live only by taking shelter of the time occupied by one of His breaths." The impersonalists do not believe in the form of the Lord, and thus they would hardly believe in the Lord's sleeping. Their idea is obtained by a poor fund of knowledge; they calculate everything in terms of man's capacity. They think that the existence of the Supreme is just the opposite of active human existence; because the human being has senses, the Supreme must be without sense perception; because the human being has a form, the Supreme must be formless; and because the human being sleeps, the Supreme must not sleep. Śrīmad-Bhāgavatam, however, does not agree with such impersonalists. It is clearly stated herein that the Supreme Lord rests in yoga-nidrā, as previously discussed. And because He sleeps, naturally He must breathe, and the Brahma-saḿhitā confirms that within His breathing period innumerable Brahmās take birth and die.
There is complete agreement between Śrīmad-Bhāgavatam and the Brahma-saḿhitā. Eternal time is never lost along with the life of Brahmā. It continues, but it has no ability to control the Supreme Personality of Godhead because the Lord is the controller of time. In the spiritual world there is undoubtedly time, but it has no control over activities. Time is unlimited, and the spiritual world is also unlimited, since everything there exists on the absolute plane.